Abstract (English)

The present dissertation deals with novel carbene catalyzed processes for C–C and C–O bond formation. Chapter 1 covers (chemo- and/or stereo-)selective nucleophilic acylation reactions between aldehydes and carbonyl electrophiles. After a brief introduction (chapter 1.1), a highly enantioselective (homo-)benzoin condensation is described in chapter 1.2. A pyroglutamic acid-derived, bifunctional carbene equipped with a protic, substituent R effectuates good to high yields and excellent ee values of the homocoupling products. The presence of a pentafluorophenyl substituent adjacent to the carbene carbon was demonstrated to be prerequisite to ensure a high catalytic activity.The extension to crossed benzoin-type condensations is presented in chapter 1.3: based on a synergism between a highly reactive catalyst system and substrate control, chemoselective cross condensations between aliphatic and ortho-substituted benzaldehydes were accomplished. Contrary to previous alternative methods, a prefunctionalization of substrates is not required. The o-substituent ensures high selectivity and can be smoothly removed in case of bromine, thus serving as a temporary and traceless directing group.
Chapter 1.4 tries to shed light on the interplay between catalyst and substrate control in crossed acyloin condensation reactions. For this purpose, the relative performance of an achiral pentafluorphenyl-substituted carbene (our “best” catalyst) and a thiazolium-derived carbene, previously introduced by Stetter, was evaluated. In cross acyloin condensations between alpha-branched aliphatic aldehydes and benzaldehydes employing achiral triazolium precatalyst, moderate to good selectivities towards cross products can be achieved without ortho-substitution of the aromatic aldehyde. One of the hitherto rare examples of a carbene catalyzed cross coupling process between aldehydes and ketones is disclosed in chapter 1.5. Pursuing a rational mechanistic approach, the novel, bioinspired method provides a modular access to alpha-hydroxy beta-ketoesters. The substrate scope – all employing readily available compounds – is remarkable, ranging from aliphatic to aromatic aldehydes, as well as differently substituted alpha-ketoesters.
Chapter 2 is devoted to carbene catalyzed C–O bond forming reactions. In particular, new NHC catalyzed lactonization strategies either relying on an internal redox reaction or external oxidation of the corresponding aldehyde substrates are presented. A carbene catalyzed access to 3,4 dihydro¬coumarins via redox lactonization of the corresponding cinnamaldehyde derivatives is presented in chapter 2.2. The transformation involves a homoenolate protonation-cyclization sequence and gives rise to lactone products in moderate to good yields. As a result of a competing oxidative pathway, coumarins are observed in minor amounts. Finally, a carbene catalyzed oxidative lactonization process was applied to the synthesis of benzodioxepinones (chapter 2.3). In this non-high dilution procedure, FeCl3 is utilized as formal terminal oxidant. Seven membered lactones are formed in good to excellent yields, selected examples exhibit interesting properties as odorants.